Sentence Correction

Souce: Kaplan GMAT 800

The average income for a person with a bachelor of arts degree is several thousand dollars per year higher than somebody with a high school diploma only.
(A) than somebody with a high school diploma only
(B) than that for somebody with only a high school diploma
(C) than for somebody with a high school diploma only
(D) than it is for somebody with a high school diploma only
(E) than that of somebody having only a high school diploma

The official answer is (B)

My question is about (D), what is wrong with it?

The official explaination suggests that "that" is vague while "it" is the right choice. Can someone explain why this is the case? Why cant we refer back to "average salary" with pronoun "it"?

I can understand (D) not being the perfect choice for other reasons like the use of a "only" at the end, but the official explanation seems confusing to me.

Is the following sentence wrong for the same official reason?
The average salary in Michigan is far lower than it is in Texas.

It refers to "average salary" here as well.

I'm just venturing a guess here, but...

in your example, as well as in the question, one could argue that the "it" has an unclear antecedent. It could (theoretically) refer to Michigan, rather than "average salary".

On the other hand, in choice B, using "that" can only refer to "the average income."

I would probably have gotten B by POE, but I'm not sure what the actual "rule" is.

IMO B

for... than that for... is parallel structure,

Going back to the official reasoning - My concern is with the rationale that "it" is vague and "that" is not.

I noticed someone mention parallelism - though not sited as the official reason, it could very well be a reason.

My doubt is for (C) option
here if we tweak the sentence to
"than for somebody with only a high school diploma . "

Does it become most correct answer , Please correct me if iam wrong

IMO no you need that to refer back to the salary

LEt me try to explain.

We are comparing Average salary here.

That correctly refers to average salary.

"It is" introduces a clause when u need a noun and breaks a rule of parallelism

when u use it is, you are essentially doing wrong comparison between "The average income for a person" and "Having Average income for somebody"

with a high school diploma only - I am not comfortable with this part of the sentence too.

myohmy?